Since portable electronic devices, hybrid vehicles, etc., have had higher performance in recent years, secondary batteries (particularly lithium-ion secondary batteries) used for such devices are increasingly required to have higher capacity. However, for current lithium-ion secondary batteries, the development of higher-capacity cathodes lags behind the development of higher-capacity anodes. Even lithium nickel oxide-based materials, which are said to have a relatively high capacity, have a capacity of only about 190 to 220 mAh/g.
Metal sulfides have relatively high theoretical capacities. Among titanium sulfides that are known as electrode materials for lithium secondary batteries, TiS2 and TiS3 have been reported to have a discharge capacity of about 240 mAh/g and about 350 mAh/g, respectively (NPL 1 and 2).
Among metal sulfides, not only titanium sulfides but also niobium sulfides have been reported to be used as cathode materials. For example, NbS2 has been reported to be capable of reversibly charging and discharging for an approximately one-electron reaction per composition formula (NPL 3 and 4). This capacity corresponds to a charge-discharge capacity of about 170 mAh/g per weight of NbS2. NbS3 also has been reported to be capable of reversibly charging and discharging for a two-electron reaction at a voltage of approximately 2 V (NPL 3 and 5). This capacity corresponds to a charge-discharge capacity of about 300 mAh/g per weight of NbS3. Further, these niobium sulfides can reversibly charge and discharge even at low voltage levels and also provide a larger capacity (a three-electron reaction, about 450 mAh/g), but the average discharge potential decreases. In addition, there is a report that NbS3 had poor reversibility and significantly deteriorated. Furthermore, the capacity actually measured was not as high as the reported value and not considered to be sufficient.
Such titanium sulfides and niobium sulfides, however, contain no lithium. Therefore, to produce a lithium-ion secondary battery, it is necessary to use a lithium-containing material as an anode to start charge and discharge from discharge. This requires a lithium-containing anode material. However, there have been only a few reports on lithium-containing anode materials for practical use.
Accordingly, using a lithium-containing material as a cathode active material is desirable. The development of a lithium-containing cathode active material capable of high-capacity charge and discharge has been desired.
As lithium-containing titanium sulfides, LixTiS2 (0≤x≤1), LixTiS3 (0≤x≤3), etc., have been reported as discharge products of TiS2 and TiS3, which are crystals that have a layered structure (NFL 1 and 6). When a large amount of lithium is intercalated into such titanium sulfides, the layered structure becomes unstable and the original structure cannot be maintained. Therefore, more than a certain amount of lithium cannot be intercalated or removed. Accordingly, to obtain a higher capacity, there is a need to develop a material that has a non-layered crystal structure.
For example, materials with a cubic crystal spinel structure have been reported as three-dimensional structures. With respect to TiS2, chemical or electrochemical intercalation of lithium into TiS2 having a cubic crystal spinel structure has been reported to produce a cubic lithium titanium sulfide (NFL 7). Specifically, NPL 7 reports that when lithium is chemically intercalated into TiS2.05 having a cubic crystal spinel structure by using normal butyl lithium, LixTi2.05S4 (0≤x≤1.95) is obtained, and that when lithium is electrochemically intercalated by using an electrochemical cell, a spinel lithium, titanium sulfide of LixTi2.05S4 (wherein 0≤x≤1.8) is obtained. It can be understood from these reports that intercalation of lithium into spinel TiS2 can produce lithium titanium sulfide of a spinel structure, which is represented by LixTi2S4 wherein x is less than 2.
In the spinel structure, however, the number of sites that may be occupied by lithium ions is approximately the same as that of transition metals. Accordingly, even if all the lithium is involved in charge and discharge, the maximum capacity is 225 mAh/g. For a higher capacity, the development of a material, such as a material having a rock salt crystal structure, which can be expected to have a higher lithium content per weight, has been desired. However, there is no report on the development of such a material.
Further, there are only a few reports on lithium-containing niobium sulfide. In particular, there is no report about high-capacity lithium-containing niobium sulfides.